Natural materials, such as bone, teeth, shells, and wood, show outstanding properties despite being porous and made of weak constituents. The reason usually lies in their sophisticated hierarchical architecture ranging from nano/microscopic to macroscopic levels. Such architectures have been perfected over the past billions of years, resulting in materials that are very often strong, tough, and lightweight, and serve as a source of inspiration for every materials designer.
Porous ceramic structures, in particular, are desirable for a wide range of applications in areas such as supported catalysis, scaffolds for tissue engineering, foams, fuel cell electrodes, filters for water purification, and many others. Multiple techniques, such as replica, direct foaming, or sacrificial templating, have been developed to manufacture such scaffolds. Most recently, three-dimensional (3D) printing has also been used as an alternative technique. However, these techniques have several limitations because they are often time-consuming or size-limiting processes, not environmentally friendly, too costly, or do not allow precise control over the final structure. An ideal strategy for engineering pores in materials in a more controllable way and at a larger scale has yet to be developed.
Freeze casting can overcome many of these previous limitations. This promising technique enables assembly of ceramic particles into scaffolds that have a highly aligned 3D porous network. The technique uses lamellar ice crystals as a template to assemble building blocks for making biomimetic scaffolds or composites. It offers the advantage of being applicable to a wide spectrum of materials (e.g., such as ceramics, polymers, and/or metals) having various shapes (e.g., particles, nanowires, and ceramic platelets, or graphene sheets). In addition, the technique is environmentally friendly, with water usually being used as the solvent. Finally, easy control of the structural features at multiple length scales is achievable by modifying ice crystal morphology with additives and/or the cooling rate.
Nevertheless, in the case of conventional freeze casting (also referred to as “ice templating” or “unidirectional freezing”), the slurry starts freezing under a single temperature gradient, causing the nucleation of ice to occur randomly on the cold finger surface. As a result, multiple small-size (e.g., submillimeter scale) domains, that is, various ice crystal orientations in the plane perpendicular to the freezing direction, are observed. Despite a pressing demand for the development of new processing techniques that can build large-scale porous aligned lamellar structures, this limitation severely hinders the scale-up fabrication of layered structures aimed for larger applications.